1. Field of the Invention
The invention relates to, in some aspects, the geometry of a stem for implantation within long bones during joint arthroplasty, particularly as it relates to Total Hip Arthroplasty or Total Shoulder Arthroplasty.
2. Description of the Related Art
Total joint arthroplasty is an established surgical treatment of diseased joints and is effective in the relief of pain and the restoration of function of disease compromised joints. Successful joint arthroplasty results in the restoration of functionality, mobility, and quality of life to patients who would otherwise be disabled as a result of joint disease.
A typical surgical procedure involves the surgical exposure of the affected joint, resection and excision of diseased bone tissue and the replacement of the excised tissue with a manufactured, mechanical joint set. The joint components may be attached to native bone by means of conventional fasteners, bone cements or interference fitting of the components within prepared cavities in the native bone structure.
A typical joint set comprises a rigid stem placed within the medullary canal of the long bone of the joint, typically manufactured from stainless steel, titanium, chromium cobalt alloy or other bio-compatible metal, a generally spherical or convex articulating component, typically manufactured of a metallic alloy or ceramic material and a corresponding concave articulating surface, typically manufactured from a polymer or ceramic material.
Exposure of the joint structure may be accomplished by a surgical approach which is directed posteriorly or by an approach which is directed anteriorly to the femur or humerus. In hip surgery the posterior approach is currently used by many surgeons as it allows for a wide exposure of the bone structures and good visualization within the wound. In shoulder surgery an anterior approach is typical. In many instances the surgical approach is dictated by the size and geometry of the implant devices being used. The current approaches typically result in a large incision size with associated muscle dissection and may be disruptive to patient anatomy. Recently, various minimally invasive techniques have been developed which require smaller incision sizes. An anterior approach to the hip joint structure has been developed and attempts are being made to minimize the incision and exposure of the shoulder joint. Associated with these techniques is a reduced exposure of, and access to, the bones within the joint structure. In addition, the reduced exposure and access pose substantial challenges to the surgeon as they relate to resecting diseases of the bone, preparing the native bone to receive implant devices, and inserting many of the implant devices currently available. The degree of difficulty associated is often higher, can result in higher complication rates when compared to conventional procedures with larger surgical exposures, and may require extended surgery times. Further, surgical outcomes are varied, depending upon surgeon skill and the nature of the defects in the native bone. As a consequence there is a need for implant components, particularly long bone implant components, which are compatible with smaller incisions and more restricted access to the native bone.
A further consequence of the small exposure is the limited space available to the surgeon within the joint wherein inter-operative adjustment of implant position can be made.
The axial position of the stem within the femoral or humeral bone is of particular importance. Misalignment of the stem with respect to the natural axis of the bone (known as varus and valgus mal-alignment) can result in abnormal biomechanics of the joint and atypical stress distributions within the bone structure which may result in implant loosening, dislocation, pain, or may cause fractures of the bone. Mal-alignments may result from poor surgical technique, inadequate exposure of the femur, incompatibility of instruments and implants with the surgical approach, or from natural deformities of the native bone within the medullary canal or metaphyseal regions of the bone.
Mal-alignment of the implanted stem may result in aseptic loosening of the implant or localized remodeling of the native cortical bone over time, ultimately causing failure of the implant and requiring additional revision surgeries to be performed to repair or replace the implant. These problems are well described and discussed in the published literature, including but not limited to, Panisello et al., (Journal of Orthopedic Surgery; 2006 April; 14(1): 32-37), Braun et al. (OrthoSupersite; October 2007), Ozturk et al. (Jun. 3, 2010), U.S. Patent Pub. No. 2004/0107002 to Katsuya, U.S. Patent Pub. No. 2008/0091274 to Murphy, U.S. Pat. No. 5,314,489 to Hoffman et al., and U.S. Pat. No. 5,514,184 to Doi et al., the descriptions of the clinical issues associated with stem alignment, biomechanics and implant failure being incorporated herein by reference in their entireties.
The aforementioned patents, publications, and articles incorporated by reference contain therein various designs and methods directed at addressing the previously discussed issues.
Various embodiments of implant stems have been described in the prior art in an attempt to resolve the issues previously described. Certain conventional stems have a proximal portion designed to locate within the metaphysis of the femur or of the humerus and a distal portion disposed to be located within the medullary canal of the bone. The proximal portions of these stems are either generally cylindrical or rectilinear (rectangular, triangular or trapezoidal) in shape when viewed in cross section along the proximal to distal axis of the implant and are intended to generally fill the metaphyseal region of the bone, the distal portion of each embodiment being narrow forms and being elongated so as to fit within the intramedullary canal of the distal region bone.
In general, the design of currently available implant stems involves the axial cross sections of the implant along the axis from the proximal portion to the distal end wherein the cross sectional profile is constant but the cross sectional area of the implant taken in a plane substantially perpendicular to the longitudinal axis reduces from the proximal to distal end.
Further, the cross sections are generally wider at the medial side than at the lateral side all along the axis of the implant. The distal portions of some implants may be cylindrical and may incorporate a variety of features including protrusions or secondary components in order to center the distal portion within the medullary canal and aid with implant alignment and fixation.
Disclosed herein is a novel design for a stem to be used within long bones during joint repair surgery.